CATHODIC PROTECTION OF STEEL BY ELECTRODEPOSITED ZINC-NICKEL ALLOY COATINGS

The ability of electrodeposited zinc-nickel alloy coatings to cathodic ally protect steel was studied in dilute chloride solutions. The poten tial distribution along steel strips partly electroplated with zinc-ni ckel alloys was determined, and the length of exposed steel that was h eld below the minimum protection potential (E(prot)) was taken as a me asure of the level of cathodic protection (CP) provided by the alloy c oatings, The level of CP afforded by zinc alloy coatings was found to decrease with increasing nickel content. When nickel content was incre ased to approximate to greater than or equal to 21 wt%, no CP was obta ined. Surface analysis of uncoupled zinc-nickel alloys that were immer sed in sodium chloride (NaCl) solutions showed the concentration of zi nc decreased in the surface layers while the concentration of nickel i ncreased, indicating that the alloys were susceptible to dezincificati on. The analysis of zinc-nickel alloy coatings on partly electroplated steel strips that were immersed in chloride solution showed a signifi cantly higher level of dezincification than that found for uncoupled a lloy coatings. This effect accounted for the rapid loss of CP afforded to steel by some zinc alloy coatings, particularly those with high in itial nickel levels.